JP6067103B2 - Working fluid composition for refrigerator - Google Patents

Description

  The present invention relates to a working fluid composition for a refrigerator. The "refrigerator" in the present invention includes an automobile air conditioner, a dehumidifier, a refrigerator, a refrigerator / freezer warehouse, a vending machine, a showcase, a cooling device in a chemical plant, a residential air conditioner, a packaged air conditioner, a hot water supply Heat pumps and the like are included.

  In the field of refrigeration and air conditioning, currently, 1,1,1,2-tetrafluoroethane (R134a), which is hydrofluorocarbon (HFC), and difluoro are used as refrigerants for refrigerators, car air conditioners, room air conditioners, and industrial refrigerators. R410A, which is a mixed refrigerant whose mass ratio of methane (R32) and pentafluoroethane (R125) is 1/1, is widely used. However, although these HFC refrigerants have an ozone depletion potential (ODP) of zero, their global warming potential (GWP) is as high as 1000 or more, so their use is restricted by so-called F-gas regulations aimed at protecting the global environment. Come.

  Tetrafluoropropene is considered as a candidate for a refrigerant having a high GWP because of its thermodynamic characteristics. The GWP of 2,3,3,3-tetrafluoropropene (HFO-1234yf) is as low as 4 and has been studied as a promising candidate.

  Regarding the base oil of general industrial lubricating oil, Patent Document 1 includes an alcohol component containing 90% by mass or more of trimethylolpropane, a monovalent fatty acid having 8 to 12 carbon atoms and adipic acid, and caprylic acid and Lubricating oil group comprising a synthetic ester obtained by reacting a monovalent fatty acid having 8 to 12 carbon atoms containing 90% by mass or more of capric acid and / or a carboxylic acid component containing 90% by mass or more of adipic acid in total. Oil has been proposed.

JP 2012-102235 A

  However, since the load conditions at the sliding part of the refrigeration equipment will become more severe in the future, there is a need for a working fluid with excellent wear resistance when tetrafluoropropene refrigerant coexists and dissolves in refrigeration oil. Yes.

  The present invention has been made in view of the above problems, and can maintain a thick oil film even under severe lubrication conditions that occur when refrigerant coexists and dissolves in refrigeration oil, has a large wear resistance effect, and is excellent in long-term reliability. It is an object to provide a working fluid composition for a refrigerator.

  The inventors of the present invention have prepared a complex ester synthesized from a specific polyhydric alcohol, polybasic acid, monohydric alcohol or monohydric fatty acid, and an ester containing a polyol ester synthesized from a specific polyhydric alcohol and a monohydric fatty acid. Refrigeration oils based on oil and tetrafluoropropene as a refrigerant, and a refrigerant working fluid for refrigeration and air conditioning that has a high refrigerant dissolution viscosity under specific conditions forms a thick oil film and exhibits high wear resistance. The present invention has been completed.

That is, this invention provides the working fluid composition for refrigerators as described in following [1]-[6].
[1] (A) at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythritol, a polybasic acid having 6 to 12 carbon atoms, and a monohydric alcohol or carbon having 4 to 18 carbon atoms A complex ester synthesized from a monohydric fatty acid having a number of 4 to 18, and (B) at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol, and the number of carbon atoms A mixed ester obtained by mixing a polyol ester synthesized from a monovalent fatty acid of 4 to 18 at a mass ratio of (A) complex ester / (B) polyol ester of 5/95 to 95/5 is contained as a base oil. Refrigeration oil,
Tetrafluoropropene as a refrigerant,
Containing
A working fluid composition for a refrigerator, wherein the refrigerant melt viscosity at a temperature of 80 ° C. and an absolute pressure of 1.6 MPa is 1.5 mm ² / s or more.
[2] The working fluid composition for a refrigerator according to [1], wherein the polyhydric alcohol constituting the (A) complex ester is neopentyl glycol and / or trimethylolpropane.
[3] The working fluid composition for a refrigerator according to [1] or [2], wherein the polybasic acid constituting the (A) complex ester is adipic acid and / or sebacic acid.
[4] The working fluid composition for a refrigerator according to any one of [1] to [3], wherein the monohydric alcohol constituting the (A) complex ester is an alcohol having 8 to 10 carbon atoms.
[5] Any one of [1] to [4], wherein the (B) polyol ester is an ester synthesized from neopentyl glycol and / or pentaerythritol and a monovalent fatty acid having 4 to 9 carbon atoms. The working fluid composition for a refrigerator according to Item.
[6] The polyol ester (B) is a mixed acid of pentaerythritol, a monovalent fatty acid having 4 carbon atoms and 3,5,5-trimethylhexanoic acid, a monovalent fatty acid having 5 carbon atoms, and 3,5,5- An ester synthesized from a mixed acid of trimethylhexanoic acid or a mixed acid of a monovalent fatty acid having 4 carbon atoms, a monovalent fatty acid having 5 carbon atoms and a mixed acid of 3,5,5-trimethylhexanoic acid. The working fluid composition for a refrigerator according to any one of [1] to [5].

  The working fluid composition for a refrigerator of the present invention has high wear resistance and good stability even under severe lubrication conditions in which the tetrafluoropropene refrigerant dissolves and the viscosity of the refrigerator oil decreases, and the equipment is stable over a long period of time. Demonstrate the special effect that can be used.

  Hereinafter, preferred embodiments of the present invention will be described in detail.

The working fluid composition for a refrigerator according to an embodiment of the present invention includes (A) at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythritol, and a polybasic acid having 6 to 12 carbon atoms. And a complex ester synthesized from a monohydric alcohol having 4 to 18 carbon atoms or a monohydric fatty acid having 4 to 18 carbon atoms, and (B) selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol. A polyol ester synthesized from at least one polyhydric alcohol and a monovalent fatty acid having 4 to 18 carbon atoms, the mass ratio of (A) complex ester / (B) polyol ester being 5/95 to 95 / Refrigerating machine oil containing a mixed ester mixed in 5 as a base oil, and a refrigerant Contains a tetrafluoropropene, a temperature of 80 ° C., it is those refrigerant solution viscosity is more than 1.5 mm ² / s at an absolute pressure 1.6 MPa.

  The complex ester has a characteristic that the oil film can be kept thick although the refrigerant is less soluble than the conventional refrigeration oil and has low compatibility with the refrigerant. Moreover, said polyol ester has favorable compatibility with a refrigerant | coolant. In the present embodiment, by blending complex ester and polyol ester having different characteristics (particularly refrigerant solubility) in this way, the oil film at the time of refrigerant coexistence can be made thicker, and thus the wear resistance of the working fluid is increased. can do.

  The complex ester has a high molecular weight, so it becomes an ester with high viscosity, so it is not compatible with the refrigerant, and as a base oil for refrigerating machine oil that must be compatible with the refrigerant from the point of oil return to the compressor. It is not suitable for use alone. One of the features of this embodiment is that the properties can be balanced by mixing the complex ester with an oil having good compatibility with the refrigerant, such as the above polyol ester.

A preferable kinematic viscosity of the complex ester is 20 to 500 mm ² / s at 40 ° C., a more preferable viscosity is 40 to 400 mm ² / s, and a further preferable viscosity is 50 to 300 mm ² / s. Further, the viscosity index is preferably 100 or more, particularly 110 to 160.

As a method for synthesizing complex esters,
(A) A method in which the molar ratio between the polyhydric alcohol and the polybasic acid is adjusted to obtain an ester intermediate in which the carboxyl group of the polybasic acid remains, and the carboxyl group is esterified with a monohydric alcohol (b) the polyhydric alcohol and the polybasic acid Examples thereof include a method in which the molar ratio of the basic acid is adjusted to obtain an ester intermediate in which the hydroxyl group of the polyhydric alcohol remains, and the hydroxyl group is esterified with a monovalent fatty acid. The complex ester obtained by the method (b) is slightly inferior in stability to the complex ester obtained by the method (a) because a relatively strong acid is generated when hydrolyzed during use as a refrigerating machine oil. It will be. As the complex ester in the present embodiment, a complex ester obtained by the method (a) having higher stability is preferable.

  As the polyhydric alcohol constituting the complex ester, neopentyl glycol and trimethylolpropane are preferable in order to obtain a viscosity suitable as a base oil. In addition, when tetravalent pentaerythritol is used, compared with neopentyl glycol and trimethylol propane, the viscosity of the resulting complex ester tends to be high and the low-temperature characteristics tend to be inferior. Furthermore, neopentyl glycol which can adjust viscosity widely is more preferable.

  The polyhydric alcohol constituting the complex ester further contains a dihydric alcohol having 2 to 10 carbon atoms other than neopentyl glycol in addition to at least one selected from neopentyl glycol, trimethylolpropane, and pentaerythritol. Then, the lubricity can be improved, which is preferable. Examples of the dihydric alcohol having 2 to 10 carbon atoms other than neopentyl glycol include ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, 2-methyl-1,3-propanediol, 3-methyl-1, 5-pentanediol, 2,2-diethyl-1,3-pentanediol and the like, butanediol having a well-balanced characteristic of the synthesized base oil is preferable, butanediol is 1,2-butanediol, 1,3 -Butanediol, 1,4-butanediol, 2,3-butanediol and the like are preferable, but 1,3-butanediol and 1,4-butanediol are more preferable in view of characteristics. The dihydric alcohol having 2 to 10 carbon atoms other than neopentyl glycol is 1.2 mol or less, particularly 0.8, with respect to 1 mol of polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythritol. It is preferably not more than mol, more preferably not more than 0.4 mol.

  The polybasic acid constituting the complex ester is a polybasic acid having 6 to 12 carbon atoms. Examples of such polybasic acids include adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, trimellitic acid, and the like. Sebacic acid is preferred, and adipic acid is more preferred. This polybasic acid is 0.4 to 4 mol, particularly 0.5 to 3 mol, more preferably 0.6 to 1 mol of polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythritol. Mole to 2.5 mol is preferred.

  In the complex ester intermediate produced by the reaction between the polyhydric alcohol and the polybasic acid, when a carboxyl group remains, the carboxyl group is esterified with a monohydric alcohol having 4 to 18 carbon atoms. Examples of the monohydric alcohol having 4 to 18 carbon atoms include linear or branched butanol, linear or branched pentanol, linear or branched hexanol, linear or branched heptanol, linear or branched octanol, linear Or aliphatic alcohols, such as branched nonanol, linear or branched decanol, linear or branched dodecanol, and oleyl alcohol. From the balance of properties, it is preferably a monohydric alcohol having 6 to 10 carbon atoms, particularly 8 to 10 carbon atoms. Among them, 2-ethylhexanol and 3,5,5-trimethylhexanol are good in the synthesized complex ester. It is preferable from the viewpoint of low temperature characteristics.

  Moreover, in the complex ester intermediate produced | generated by reaction of said polyhydric alcohol and polybasic acid, when a hydroxyl group remains, the said hydroxyl group is esterified with a C1-C18 monohydric fatty acid. Examples of the monovalent fatty acid having 4 to 18 carbon atoms include linear or branched butanoic acid, linear or branched pentanoic acid, linear or branched hexanoic acid, linear or branched heptanoic acid, linear or branched octane Examples thereof include acid, linear or branched nonanoic acid, linear or branched decanoic acid, linear or branched dodecanoic acid, and oleic acid. Preferred are monovalent fatty acids having 8 to 10 carbon atoms, among which 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid are preferred from the viewpoint of low temperature characteristics.

  On the other hand, the polyol ester in this embodiment is synthesized from at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol, and dipentaerythritol, and a monovalent fatty acid having 4 to 18 carbon atoms. Polyol ester.

  As the polyhydric alcohol constituting the polyol ester, trimethylolpropane and pentaerythritol are preferable, and pentaerythritol is more preferable from the viewpoint of balance of characteristics.

  Examples of the monovalent fatty acid having 4 to 18 carbon atoms constituting the polyol ester include linear or branched butanoic acid, linear or branched pentanoic acid, linear or branched hexanoic acid, linear or branched heptanoic acid, direct Examples include chain or branched octanoic acid, linear or branched nonanoic acid, linear or branched decanoic acid, linear or branched dodecanoic acid, and oleic acid. From the viewpoint of low-temperature characteristics, it is preferably a monovalent fatty acid having 4 to 9 carbon atoms, particularly branched butanoic acid, branched pentanoic acid, branched hexanoic acid, branched heptanoic acid, 2-ethylhexanoic acid and 3,5 , 5-trimethylhexanoic acid is more preferred. In particular, a saturated monovalent fatty acid having 4 or 5 carbon atoms is preferred.

  Most preferable from the viewpoint of compatibility with the refrigerant, the polyhydric alcohol is pentaerythritol, and the monovalent fatty acid is a polyol ester synthesized from a mixed acid of a branched fatty acid having 4 to 9 carbon atoms.

  In the working fluid composition for a refrigerator according to the present embodiment, when the content of the complex ester is (A) and the content of the polyol ester is (B), (A) / (B) is mass. The ratio is 5/95 to 95/5, and 50/50 to 95/5 is preferable from the viewpoint of lubricity in order to make better use of the characteristics of each ester, and 50/50 from the viewpoint of compatibility with the refrigerant. ~ 5/95 is preferred.

The kinematic viscosity at 40 ° C. of the refrigerating machine oil in the present embodiment is preferably 3 to 500 mm ² / s, more preferably 8 to 150 mm ² / s, and still more preferably 20 to 100 mm ² / s. Further, the viscosity index of the refrigerating machine oil is preferably 50 or more, particularly preferably 80 to 120.

  The pour point of the refrigerating machine oil in the present embodiment is preferably −10 ° C. or lower, more preferably −20 ° C. or lower.

  The acid value of the refrigerating machine oil in the present embodiment is preferably 0.1 mgKOH / g or less, more preferably, in order to prevent corrosion of the metal used in the refrigerating machine or piping and to suppress deterioration of the refrigerating machine oil itself. May be 0.05 mgKOH / g or less. In addition, the acid value in this invention means the acid value measured based on JISK2501 "acid value test method".

  Moreover, the flash point of the refrigerating machine oil in the present embodiment is preferably 120 ° C. or higher, more preferably 200 ° C. or higher.

  In addition, the water content of the refrigerating machine oil in the present embodiment is preferably 200 ppm or less, more preferably 100 ppm or less, and most preferably 50 ppm or less. In particular, when used in a hermetic refrigerator, the moisture content is required to be low from the viewpoint of stability of the refrigerator oil and electrical insulation.

  Moreover, the refrigerating machine oil in this embodiment may further contain other base oils such as mineral oil base oil and synthetic base oil in addition to the complex ester and polyol ester. The total content of the complex ester and the polyol ester is preferably 80% by mass or more, particularly 95% by mass or more of the refrigerating machine oil.

  In the case of a refrigerator, as described above, from the viewpoint of preventing global warming, the current HFC refrigerant with a high GWP is moving to a refrigerant with a low GWP, and a refrigerator oil adapted to them is required. There is a need for a suitable working fluid mixed with machine oil. In the present invention, tetrafluoropropene is contained as a refrigerant, and the content of tetrafluoropropene in the refrigerant is preferably 60 to 100% by mass, particularly preferably 80 to 100% by mass. As tetrafluoropropene (HFO-1234), 2,3,3,3-tetrafluoropropene (HFO-1234yf), 1,3,3,3-tetrafluoropropene (HFO-1234ze), 1,2,3 , 3-tetrafluoropropene (HFO-1234ye) and the like can be used.

  The blending ratio of the refrigerating machine oil composition and the refrigerant in the working fluid composition for a refrigerating machine according to the present embodiment is not particularly limited, but the refrigerating machine oil composition is preferably 1 to 500 parts by mass with respect to 100 parts by mass of the refrigerant. Preferably it is 2-400 mass parts.

The refrigerant viscosity at a temperature of 80 ° C. and an absolute pressure of 1.6 MPa of the working fluid composition for a refrigerator according to the present embodiment is 1.5 mm ² / s or more, preferably 1.7 mm ² / s or more, Usually, it is 15.0 mm ² / s or less.

  The working fluid composition according to the present embodiment can contain various additives in order to further improve the wear resistance. Suitable additives include phosphate esters, and particularly preferred compounds are triphenyl phosphate (TPP) and tricresyl phosphate (TCP).

  A suitable sulfur-based additive is sulfide, and there are various types of sulfide compounds, but monosulfide compounds are preferable. For example, a highly active sulfur compound such as a disulfide compound deteriorates the stability of refrigerating machine oil and alters the copper used in the refrigeration equipment.

  The working fluid composition for a refrigerator according to the present embodiment includes, in addition to the above-described additives, an antioxidant and a friction modifier that have been conventionally used in lubricating oils as long as the object of the present invention is not impaired. Additives such as antiwear agents, extreme pressure agents, rust preventive agents, metal deactivators, and antifoaming agents can be included to further improve performance.

  Antioxidants include di-tert. A phenolic compound such as butyl-p-cresol and an amine compound such as alkyldiphenylamine can be contained. In particular, the phenolic compound antioxidant is preferably contained in an amount of 0.02 to 0.5% by mass based on the total amount of refrigerating machine oil.

  Extreme pressure agents such as aliphatic amines, aliphatic amides, aliphatic imides, alcohols, esters, acidic phosphate ester amine salts, phosphite ester amine salts as friction modifiers, zinc dialkyldithiophosphates as antiwear agents As sulfurized olefins, sulfurized fats and oils, rust preventives such as alkenyl succinic acid esters or partial esters, metal deactivators such as benzotriazole and benzotriazole derivatives, antifoaming agents such as silicone compounds and polyester compounds Each is listed.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example at all.

[Examples 1-8, Comparative Examples 1-3]
In Examples 1 to 8 and Comparative Examples 1 to 3, first, base oils having compositions shown in Tables 1 and 2 were prepared using the following base materials.
[A] Complex ester (A-1) An ester intermediate obtained by reacting neopentyl glycol (1 mol) and 1,4-butanediol (0.3 mol) with adipic acid (2.4 mol), , 5,5-trimethylhexanol (2.5 mol) and the remaining unreacted product was removed by distillation (kinematic viscosity at 40 ° C. 67.8 mm ² / s, viscosity index 145).
(A-2) An ester intermediate obtained by reacting trimethylolpropane (1 mol) and 1,3-butanediol (0.2 mol) with sebacic acid (2.4 mol) is further mixed with normal heptanol (1. 6 mol) is reacted, and the remaining unreacted product is removed by distillation (kinematic viscosity at 40 ° C. 77.3 mm ² / s, viscosity index 148).
(A-3) 2-ethylhexanol (2.0 mol) was further reacted with an ester intermediate obtained by reacting adipic acid (2.4 mol) with trimethylolpropane (1 mol), and the remaining unreacted product Was obtained by distilling off an ester (kinematic viscosity at 40 ° C. 68.8 mm ² / s, viscosity index 120).
(A-4) An ester intermediate obtained by reacting neopentyl glycol (1 mol) with adipic acid (0.8 mol) is further reacted with 3,5,5-trimethylhexanoic acid (0.5 mol), An ester obtained by removing the remaining unreacted substance by distillation (kinematic viscosity at 40 ° C. 71.5 mm ² / s, viscosity index 114).
[B] Polyester (B-1) An ester of pentaerythritol and a mixed acid of 2-methylpropanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 35:65 (kinematic viscosity at 40 ° C. 69. 4 mm ² / s, viscosity index 95)
(B-2) Esters of pentaerythritol, 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 45:55 (kinematic viscosity at 40 ° C. 70.1 mm ² / s, Viscosity index 90)
(B-3) Neopentyl glycol and ester of 2-ethylhexanoic acid (kinematic viscosity at 40 ° C. 8.3 mm ² / s, viscosity index 56)
(B-4) An ester of pentaerythritol, pentanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 40:60 (kinematic viscosity at 40 ° C. 53.8 mm ² / s, viscosity index 99 )

  Of these base oils, the synthesis reaction of the esters [A] and [B] was carried out without using a catalyst and a solvent, and a trace amount of impurities was removed by adsorption treatment (white clay treatment) in the final step. The kinematic viscosity and the viscosity index were measured and calculated according to JIS K2283.

  Next, the base oils of Examples 1 to 8 and Comparative Examples 1 to 3 were mixed with di-tert. Refrigerating machine oil was prepared by blending 0.1% by mass of butyl-p-cresol (DBPC) in each case.

  About the refrigerating machine oil of Examples 1-8 and Comparative Examples 1-3, the melt viscosity was measured as follows.

(Measurement of refrigerant melt viscosity)
100 g of refrigerating machine oil is put into a 200 ml pressure vessel containing a vibration type viscometer, the inside of the vessel is vacuum degassed, and then a HFO-1234yf refrigerant is put into a working fluid composition to prepare a working fluid composition at a temperature of 80 ° C. and an absolute pressure of 1. Viscosity was measured by adjusting the pressure of the refrigerant and the temperature of the pressure vessel under the condition of 6 MPa. The obtained result is shown to Tables 1-2.

  It turns out that the working fluid composition of Examples 1-8 has a high refrigerant | coolant melt viscosity.

  Moreover, about the refrigerating machine oil of Examples 1-8, it has confirmed that the lubricity (wear resistance) is favorable by the following lubricity tests.

(Lubricity test)
Testing machine: High-pressure atmosphere friction testing machine manufactured by Shinko Engineering Co., Ltd. (rotating sliding method with rotating vane material and fixed disk material)
Oil volume: 600ml
Test temperature: 110 ° C
Rotation speed: 500rpm
Load load: 80kgf
Test time: 1 hour Vane material: SKH-51
Disc material: FC250
Refrigerant: HFO-1234yf
Test container pressure: 1.6 MPa

  The working fluid composition for a refrigerator according to the present invention has a remarkable effect that the viscosity at the time of dissolution of the tetrafluoropropene refrigerant is high and the lubricity is greatly improved. Therefore, a refrigeration / air conditioning system with a high cooling efficiency that has a compressor, a condenser, a throttle device, an evaporator, etc., and circulates a refrigerant between them, especially a rotary type, a swing type, a scroll type compressor, etc. The present invention can be suitably used in a system including a room air conditioner, a packaged air conditioner, a refrigerator, a car air conditioner, and an industrial refrigerator.

Claims (5)

(A) at least one polyhydric alcohol selected neopentyl glycol and trimethylol propane or al, polybasic acids having 6 to 12 carbon atoms, monohydric alcohols or carbon number of 4 to 18 carbon atoms 4 A complex ester synthesized from 18 monovalent fatty acids having a kinematic viscosity at 40 ° C. of 40 to 400 mm ² / s , and (B) neopentyl glycol, trimethylolpropane, pentaerythritol and diester A polyol ester synthesized from at least one polyhydric alcohol selected from pentaerythritol and a monohydric fatty acid having 4 to 18 carbon atoms is converted into a mass ratio of 5 / (complex ester) / (B) polyol ester. Refrigeration containing mixed ester mixed at 95-95 / 5 as base oil And oil,
Containing tetrafluoropropene as a refrigerant,
A working fluid composition for a refrigerator, wherein the refrigerant melt viscosity at a temperature of 80 ° C. and an absolute pressure of 1.6 MPa is 1.5 mm ² / s or more. The working fluid composition for a refrigerator according to claim 1 , wherein the polybasic acid constituting the (A) complex ester is adipic acid and / or sebacic acid. The working fluid composition for a refrigerator according to claim 1 or 2 , wherein the monohydric alcohol constituting the (A) complex ester is an alcohol having 8 to 10 carbon atoms. The refrigeration according to any one of claims 1 to 3 , wherein the (B) polyol ester is an ester synthesized from neopentyl glycol and / or pentaerythritol and a monovalent fatty acid having 4 to 9 carbon atoms. Working fluid composition for machinery. The (B) polyol ester is pentaerythritol, a mixed acid of a monovalent fatty acid having 4 carbon atoms and 3,5,5-trimethylhexanoic acid, a monovalent fatty acid having 5 carbon atoms and 3,5,5-trimethylhexanoic acid Or an ester synthesized from any one of a mixed acid of a monovalent fatty acid having 4 carbon atoms, a monovalent fatty acid having 5 carbon atoms and a mixed acid of 3,5,5-trimethylhexanoic acid. Item 5. The working fluid composition for a refrigerator according to any one of Items 1 to 4 .